1. Any of a class of chemical compounds whose aqueous solutions turn blue litmus paper red, react with and dissolve certain metals to form salts, and react with bases to form salts.

2. A compound capable of transferring a hydrogen ion in solution.

3. A substance that ionizes in solution to yield the positive ion of the solvent.

4. A molecule or ion that combines with another molecule or ion by forming a covalent bond with two electrons from the other species.